Constant duty cycle control of induction cooking inverter

ABSTRACT

In a solid state cool-top cooking appliance for inductively heating a utensil, an inverter for driving the induction heating coil utilizes a constant duty cycle control circuit to optimize performance with the utensil coupled with and removed from the energized coil. To compensate for the increased period of oscillation when unloaded, the inverter operating frequency is automatically decreased to reduce voltage and current stresses on circuit components. Conversely, the operating frequency increases when loaded to couple more power to the utensil. An on-off sensor continuously senses an inverter circuit parameter indicative of the load condition, such as a voltage divider and associated switch for sensing the thyristor anode voltage. The sensed information is processed by the control circuit in closed feedback loop manner to vary the thyristor gating signal repetition rate. Disclosed with regard to a one-thyristor series resonant inverter, the technique is applicable to inverters generally.

I Steigerwald Dec. 25, 1973 CONSTANT DUTY CYCLE CONTROL OF INDUCTIONCOOKING INVERTER Primary Examiner-Bruce A. Reynolds Attrney-John F.Ahern et al.

[75 lnventor: Robert L. Steigerwald, Schenectady,

57 ABSTRACT [73] Asslgnee: g ompany In a solid state cool-top cookingappliance for inducc enec tively heating a utensil, an inverter fordriving the in- [22] Filed: June 28, 1972 duction heating coil utilizesa constant duty cycle control circuit to optimize performance with theutensil [21] Appl' 267d coupled with and removed from the energizedcoil. To compensate for the increased period of oscillation [52] US. Cl219/l0.49, 219/1077, 321/18 when unloaded, the inverter operatingfrequency is [51] Int. Cl. Hb 5/04 automatically decreased to reducevoltage and current [58] Field of Search 219/1049, 10.75, stresses oncircuit components. Conversely, the oper- 219/ 10.77, 10.79, 502, 501;321/10, 18 ating frequency increases when loaded to couple more power tothe utensil. An on-ofi sensor continuously [56] References Cited sensesan inverter circuit parameter indicative of the U I STATES PATENTS loadcondition, such as a voltage divider and asso- 3 200 933 M97:Haakenniden et al 219/501 ciated switch for sensing the thyristor anodevoltage. 3 593 103 7/1971 Chandler et al. .:::::I:::.. 321/18 The sensedinfrmatin is Pmessed by 3:7l0:062 1/1973 Peters 219/1049 circuit inclosed feedback loonmanner y the 3,551,632 12/1970 Germ 219/1017 xthyristor g g Signal repetition rate- Disclosed with 3,320,512 5/1967Kruger 321/ regard to a one-thyristor series resonant inverter, the3,449,629 6/1969 Wigert et aL. 2l9/502X technique is applicable toinverters generally. 3,670,234 6/1972 Joyce 321/18 12 Claims, 10 DrawingFigures 40% 3s |4 r /l/ [7r I I o c @Q? 42 l POWER A W SUPPLY I5 dc 4| 1I i 1 I min j i 33' THYRISTOR 45 VOLTAGE I 33 .ANODE AVERAGiNG 47CONTROLLED .L VOLTAGE i FILTER OSCILLATOR Z 71 I ON'OFF om CIRCUIT ANDPULSE 32 SENSOR H GENERATOR 34 I 43 46 1 i V L J PATENTEUUEEZSIBYS3.781.505

' sum 1 0F 3 STATIC POWER CONVERSION uNIT wig 5 40\3 35 I4 I" "-1 I/ Il7r I DC 1 17 l POWER A\ m I SUPPLY I5 de: 4|\ I 1 l I v l THYRISTOR 45VOLTAGE :33 33 I ANODE AVERAGING 47 CONTROLLED I j: I VOLTAGE FILTEROscILLATOR ;Z S f l ONTOFF m |RCU|T AND PULSE 32 I SENSOR GENERATOR I 34T l i 43 46 44 1 l I L i O V PATEHTED UECZ 51975 LOADED ECTION CURRENTVOLT THYRISTOR ANODE E AGE 0 SHiU 2 0F 3 THYRISTOR 33 RECHARGE CURRENTUNLOADED WITH KNOWN CONSTANT REPETITION RATE GATING Q CTToN A A 01 5CURRENT V 9 1m sTo Zj m f' VOLTAGE P UNLOADED WITH CONSTANT DUTY CYCLEGATING INDUCTION COIL CURRENT V THY ToR CSETAGE PATENTED UEE25I975 SHEET3 BF 3 CONSTANT DUTYCYCLE CONTROL OF INDUCTION COOKING INVERTERBACKGROUND OF THE INVENTION This invention relates to solid stateinverters with a constant duty cycle control circuit, and to inductioncooking appliances employing such inverters for improved performancewith the utensil on and off the energized. coil. Solid state cookingappliances utilizing the principles of induction heating are commonlyreferred to as the cool-top range and the counter-top range. To producethe alternating magnetic field that couples power to the cookingutensil, a solid state inverter operated at an ultrasonicfrequency isused to drive the induction heating coil. The utensil functions as theinverter load, and removal of the utensil from the energized coilresults in a change in the inverters electrical parameters since it thenoperates in the no-load condition. A low cost inverter for use in solidstate cooking equipment is a onethyristor series resonant circuit inwhich the induction heating coil and a commutating capacitor form thebasic high frequency oscillator. At a selected operating frequency ofthe inverter, removal of the utensil from the coil reduces the losses inthe resonant circuit. Thus, the components are subjected to greaterstresses due to the undamped nature of the resonant circuit at a timewhen'th'e circuit is in the standby mode. To withstand these. greaterstresses, the components can be selected with unnecessarily highratings. Another solution employs autensil presence detection circuitthat senses the thyristor anode voltage by a threshold technique andautomatically decreases the inverter operating frequency when anovervoltage is sensed. This is described in copending allowedapplication Ser. No. 211,926,. filed-on Dec. 27, 1971 by William P.Kornrumpf and John D. Harnden, Jr., and assigned to the same assignee asthe present invention.

Other requirements of cooking equipment to be used by technicallyunskilled persons such as housewives and. chefs are that it beautomatic, reliable, and convenient to operate. In particular, it isdesirable to have the circuit start up and shut down automatically aswell as to adapt to different load conditions. Such features are,

of course, advantageous in inverters for other types of inductionheating and other technical applications.

SUMMARY OF THE INVENTION As applied to an improvement in solid stateinduction cooking appliances, it is recognized that in appropriateinvertersthe load condition of the induction heating coil, i.e., themagnitude of the utensil load and in particular whether the coil isloaded due to coupling power to'the utensil or unloaded by the absenceof a utensil, can be detected by sensing a preselected inverter power:circuit parameter. In the case of a series resonant inverter, forexample, the period of oscillation is shorter with the utensil in'cooking position and longer when removed from the energized coil. Ingeneral, the heating of a steel utensil produces a shorter period ofoscillation than the heating of an aluminum utensil due to higher lossesin the series resonant circuit, i.e., due to bettercoupling between coiland load.

In accordance with the invention, a constant duty cycle control circuitis utilized to supply turn-on or gatingsignals to the power device ordevices controlling .the inverter to vary the inverter operatingfrequency as a function of the load condition of the induction heatingcoil. The constant duty cycle control circuit is controlled by an on-offsensor, as contrasted to a threshold type sensor, that is connectedbetween selected points on the power circuit and signal level controlcircuit and continuously senses a preselected power circuit parametersuch as the invervals of conduction and nonconduction of the powerdevice or devices or the intervals of oscillation and nonoscillation. Inclosed feedback loop fashion, the operating frequency automaticallyincreases when the coil is loaded to thereby couple more power to theutensil, and decreases with unloading of the coil by removal of theutensil to thereby reduce the peak voltages and currents for standbyoperation. The preferred form of an on-off sensor is a resistancevoltage divider circuit effectively connectedacross the terminals of thepower device or devices, such as the anode and cathode of a thyristor tosense the thyristor anode voltage, and a solid state switch coupled to ajunction of the voltage divider to produce a train of variable widthvoltage pulses whose widths are in accordance with the on-off times ofthe power device or devices. The constant duty cycle control circuitfurther preferably includes an averaging filter circuit for convertingthe train of pulses to a variable direct voltage signal, and a voltagecontrolled oscillator and pulse generator for producing gating signalsat a variable repetition rate dependent on the magnitude of the directvoltage signal. As additional features, the control circuit may includea minimum and maximum frequency control for limiting the range of thedirect voltage signal, and a start-up and shut-down circuit.

The principles of the invention are explained in regard to aone-thyristor series resonant inverter, but are applicable generally toother inverters and other applications.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammaticcross-sectional view of an induction surface cooking unit showing aconventional utensil on a cool cooking surface being heated by aninduction heating coil;

FIG. 2 is a perspective view of a portable induction warming appliancesuitable for defrosting frozen foods in aluminum foil containers and fordisposable foil cooking;

FIG. 3 is a schematic circuit diagram of the inverter power circuitshowing in block diagram form the major conpoments of the constant dutycycle control circuit;

FIGS. 4a and 4b are waveform diagrams of the induction coil current andthyristor anode voltage for a loaded inverter with the utensil coupledto the coil;

FIGS. 5a and 5b are waveform diagrams of the induction coil current andthyristor anode voltage for an unloaded inverter with the utensilremoved from the energized coil, when the inverter is operated in knownmanner with the same constant repetition rate gating control as in FIGS.4a, 4b;

FIGS. 6a and 6b are waveform diagrams of the induction coil current andthyristor anode voltage for an unloaded inverter when operated accordingto the invention with constant duty cycle gating control; and

FIG. 7 is a detailed circuit diagram of the constant duty cycle controlcircuit illustrated in simplifier form in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT The basic structural featuresofa solid state induction cooking appliance are illustrateddiagrammatically in FIG. 1. Static power conversion circuit 12 ispreferably energized by a single phase commercially available 60 Hz, 120or 240 volt source of alternating voltage, but in appropriate cases canbe energized by a battery. As used with an alternating voltage source,static power converter 12 most commonly comprises a full wave bridgerectifier and a filter network for producing a d-c supply voltage thatis converted by a solid state inverter to an ultrasonic frequencyvoltage wave for driving the induction heating coil 15. Inductionheating coil 15 is typically a single layer, annular, air-core orferromagnetic-core coil made with tightly wound braided ribbonconductors or solid flat strip conductors. In the cooking appliance,induction heating coil 15 is appropriately mounted in a horizontalposition immediately below a non-metallic or substantially non-metallicsupport plate 16', made for example of a thin sheet of glass or plastic.Plate 16 supports the metallic cooking utensil 17 to be heated, and inan electric range or built-in cooktop is referred to as the counter-topcooking surface. Cooking utensil 17 is an ordinary cooking pot or pan, afrying pan, or some other available metallic utensil used in foodpreparation, made either of magnetic or nonmagnetic materials. Operationof static power converter 12 to impress an ultrasonic frequency voltagewave on induction heating coil 15 results in the generation of analternating magnetic field which is coupled across the air gap andutensil support plate 16 to utensil 17. At an ultrasonic operatingfrequency in the range of 18-40 kHz, the cooking appliance is inaudibleto most people.

Among the advantages of induction cooking are, briefly, that thecontinuous and unbroken cooking surface 16 remains relatively cool tothe human touch; spilled foods do not burn and char,and hence, bothsurface 16 and utensil 17 are easy to clean; and the unobstructedcooking surface is immediately available for other food preparation andcooking tasks. The utensil isheated more uniformly than is the case witha conventional gas range or electric resistance heating range,

and transfer of energy to utensil 17 is relatively efficient andconsistent whether the range and utensil are new or old. Other desirableuser features are the fast utensil warm-up and rapid response to changesin the heating level; noisless operation; and complete freedom to movethe utensil on the cooking surface since at ultrasonic frequencies thereare insignificant reaction forces of either attraction or replusionacting on the utensil. An induction surface cooking unit for use withconventional pots and pans and other cooking utensils such as utensil 17in FIG. 1 is described more fully in allowed application Ser. No.200,530, filed on Nov. 19, 1971, by William P. Kornrumpf, now U.S. Pat.No. 3,697,716, dated Oct. 10, 1972, and assigned to the same assignee asthe present invention. In the domestic induction range or cooktop theredisclosed, induction heating coil 15 is a simple flat spiral coil.Further, a useful power range for general cooking is from 1 to 1.5kilowatts to a lower limit of 100 watts or less in order to perform thecommon cooking requirements ranging from rapid heating to warming.

The portable single-coil induction cooking unit shown in FIG. 2 isespecially suitable for the defrosting and warming of frozen conveniencefoods packaged in thin aluminum foil containers. The unit is also usedfor the warming and cooking of foods in a user-made disposable aluminumfoil utensil, as well as foods placed on a sheet of aluminum foil orfoil that is wrapped about the food to achieve an oven effect. Aparticular advantage of disposable foil cooking is that it obviates theclean-up and storage problems of pots and pans. The aluminum foilinduction cooker of FIG. 2 is intended to stand on a kitchen countersurface and is energized by a volt source. A relatively small box-likehousing 19 contains the electronic circuitry and induction heating coil15, and the unit is controlled by an onoff knob 21 and, if provided, apower level knob 21.. A combination leg and handle unit 22 is attachedto housing 19 to facilitate easy handling and storage. Faster and moreefficient cooking and warming results are produced by enclosing thecooking surface 16 with a removable cover 24, which conveniently is madeof a suitable transparent plastic. To obtain even heating of aluminumfoil containers and disposable utensils with a rectangular or a squareshape, such as the frozen TV dinner 17., induction heating coillSpreferably has several series-connected elongated coil sections in arectangular configuration. An evenheating induction coil for producing auniform heating distribution in aluminum foil withcan optimum thicknessof 0.5 mils is comprised by three elongated simple coil sectionsarranged side-by-side and surrounded by a peripheral coil section. Forfurther information on metallic foil induction cooking, including adescription of this coil,-the reader is referred to copending allowedapplication Ser. No. 245,570, filed Apr. 19, 1972, by John D. Harnden,Jr. and William P. Kornrumpf, and assigned to the same assignee as thisinvention. An induction surface cooking unit solely for aluminum foilcooking and warming can have a maximum power in the range of 200-400watts, and an adjustable heating level or temperature may not berequired.

Applied to a solid state induction cooking appliance, the inverter powercircuit constructed according to the teaching of the invention with aconstant duty cycle control circuit as illustrated in simplified blockdiagram fonn in FIG. 3 is useful to achieve improved operation andgreater load tolerance with the utensil coupled to and removed from theenergized coil. The effect of constant duty cycle control operation isto automatically reduce the output power available at the inductionheating coil when the circuit is in the standby mode. The preferredembodiment utilizes a onethyristor series resonant inverter, however itwill be understood that the invention is applicable in appropriate casesto other inverter configurations using other types of powersemiconductors such as transistors and other thyristors. The preferredembodiment is further described with regard to a portable aluminum foilwarming appliance, but the principles apply generally to inductionsurface cooking units with higher power ranges for domestic ranges andcooktops and for commercial cooking equipment. The inductively heatedutensil can be a conventional cooking pot or pan, or a stamped reusablethin metal multi-cavity cooker, as well as the previously describedfrozen convenience food aluminum foil container, a user-fashioneddisposable foil utensil, a sheet of foil or a foil wrapping, and so on,all of which are hereafter referred to as the utensil 17.

In order to understand the constant duty cycle control circuit, it isnecessary to first discuss briefly the structure and operation of theinverter power circuit in FIG. 3, which shows a preferred form ofinverter 14 for use in ultrasonic frequency induction cookingappliances. Reference may be made to the now US. Pat. No. 3,697,716, fora more complete description of inverter 14. This one-thyristor seriesresonant inverter requires a small number of power components, only onegating or control circuit, and the output power is easily controlled.The d-c power supply, not here illustrated, is a full wave diode bridgerectifier and a simple filter network for supplying a constant d-c inputvoltage E at inverter input terminals 30 and 31. Inverter 14 comprises aunidirectional conducting power thyristor 33 connected in series circuitrelationship with a reset inductor 35 between d-c input terminals 30 and31. A diode 34 to conduct power current in the reverse direction isconnected across the load terminals of thyristor 33. A series snubber RCcircuit 33' is also usually connected across the load terminals ofthyristor 33 to limit the rate of reapplication of forward voltage tothe device which otherwise could falsely trigger thyristor 33 into theconducting state. The basic power circuit is completed by a computatingcapacitor 32 and induction heating coil connected in series with oneanother and coupled directly across the terminals of theinverse-parallel combination of thyristor 33 and diode 34. When eitherof the power devices is conducting, capacitor 32 and induction heatingcoil 15 form a series resonant circuit for generating damped sinusoidalcurrent pulses that flow through induction heating coil 15, which hasthe dual function of providing commutating inductance as well ascoupling power to the load. Reset inductor 35 functions to resetcommutating capacitor 32 by charging the commutating capacitorpositively during the non-conducting intervals of the thyristordiodecombination. Each cycle of current flow is initiated by a gating pulseapplied to thyristor 33 by the constant duty cycle control circuit 40.

The application of a gating pulse to thyristor 33 by constant duty cyclecontrol circuit 40 causes it to turn on, energizing the high frequencyseries resonant circuit essentially comprising commutating capacitor 32and induction heating coil 15. A damped sinusoidal current pulse flowsthrough induction heating coil 15 and charges commutating capacitor 32negatively. The amount of damping depends upon the degree of loading ofinduction heating coil 15. At this point the current in the seriesresonant circuit reverses and a damped sinusoidal current pulse of theopposite polarity flows through induction heating coil 15 and diode '34.During the time that feedback diode 3 2- is conducting, thyristor 33 isreverse biased by the voltage across diode 34 and turns off. When thecurrent in the series resonant circuit again attempts to reverse,thyristor 33 does not conduct since it has regained its forward voltageblocking capabilities, and a gating pulse is not applied to thethyristor at this time. Because of the losses in the electrical circuitdue mostly to the heating of the utensil, commutating capacitor 32 atthe end of the complete conduction cycle on a steady state basis is leftcharged to a lower voltage than it had at the beginning of theoscillation. During the circuit off-time when both of the power devices33 and 34 are non-conducting, the

additional energy stored in reset inductor 35 is discharged andtransferred primarily to commutating capacitor 32, thereby rechargingthe commutating capacitor 32 to its original voltage level on a steadystate basis and thus restoring the commutating capacitor energy whichwas lost during the oscillation period During the energy transfer periodsome energy is also drawn from the d-c supply through reset inductor 35to aid in the recharging of commutating capacitor 32.

FIG. 4a shows the asymmetrical sinusoidal induction coil current forseveral complete cycles of operation for the loaded condition of theinverter, with utensil 17 coupled to the coil and being heated byinduction. The components of coil current produced during the highfrequency oscillation when conducting through thyristor 33 and diode 34are labeled. The time delay interval between successive sinusoidalpulses corresponds to the energy transfer period when there is a smallcurrent circulating in coil 15 due to the recharge current of capacitor32. With preactical component choices, the circuit transfers more energyfrom reset inductor 35 and the d-c source to commutating capacitor 32 asthe energy transfer period T-t, is made shorter relative to the highfrequency oscillation period t,. FIG. 4b shows the thyristor anodevoltage at point A (FIG. 3) with respect to the negative of the dosupply, on the inverter power circuit. The operating frequency orrepetition rate of the inverter determines the period T of each completecycle of operation. During the oscillatory interval t, when thyristor 33or diode 34 is conducting, point A, neglecting the diode or thyristorvoltage drops, is at the potential of negative d-c input terminal 31.Consequently, current is built up in reset inductor 35. During thenon-oscillatory interval T-t, corresponding to the energy transferperiod, the potential at point A, referred to the negative d-c terminal,is substantially the same as the commutating capacitor voltage and risesapproximately linearly as capacitor 32 is recharged by reset inductor 35and the d-c source. The voltages V1 and V2 are essentially the initialand final voltages on capacitor 32, and are a measure of the energy lostduring the oscillation, which is a function of coil loading. The averagevoltage appearing at the thyristor anode is equal to the d-c supplyvoltage E since, in the steady state, no d-c voltage can appear acrossreset inductor 35. Thus, in a complete cycle of operation, the shadedarea below the dashed line representing E is equal to the shaded areaabove the dashed line. When the operating frequency of thee inverter isincreased, reducing the period T, more energy is transferred from resetinductor 35 to commutating capacitor 32 during the shortened energytransfer period, with the result that the initial capacitor voltage V1is higher. There are accordingly two effects that increase the power inwatts supplied to the utensil when the inverter operating frequency isincreased. There are larger as well as more frequently applied currentpulses in induction heating coil 15.

The load for inverter 14 is provided by the electrical losses in theutensil. With respect to the utensil load, induction heating coil 15functions as the primary winding of an air-core transformer. In aphysical equivalent circuit for the utensil, identified generally at 17,the utensil functions as a single turn secondary winding with a seriesresistance 17r connected between the ends of the single turnrepresenting the IR or eddy cur rent losses, and hysteresis losses whereapplicable. The

currents and voltages induced in the utensil are determined essentiallyby the transformer laws. When there is no utensil coupled with theenergized coil, the unloaded inverter has reduced losses with aresultant increase in the higher frequency oscillation period t,. Thatis, in the loaded'condition the inverter power circuit can be describedas a damped oscillatory L-C-R circuit, whereas in the unloadedcondition, it is essentially an undamped oscillatory L-C circuit withrelatively little loss in the high frequency resonant circuit. Referencemay be made to the previously identified copending application Ser. No.21 1,926 for a further discussion of this and other common subjectmatter.

FIGS. a and 5b show the induction coil current and thyristor anodevoltage for an unloaded inverter with the same operating frequency as inFIGS. 4a and 4b. Thus, gating pulses are supplied to thyristor 33 inknown manner at a constant repetition rate for both the loaded andunloaded conditions. The period T is the same, but the high frequencyoscillation period t, increases dueprimarily to the removal of theutensil resistive losses; During the oscillatory interval, commutatingcapacitor 32 charges positively almost as far as it does negatively..lneffect, point A is at the potential of the negative d-c terminal for alonger period than the loaded case and thus a larger average positivevoltage is appliedto reset inductor 35. Since the average value ofvoltage across an ideal inductor must be zero in the steady state,capacitor 32 is charged to a much higher voltage. Also, thenon-oscillatory interval T-t, decreases when the coil is unloaded andresults in larger current pulses since the initial capacitor voltage isat a much higher level. Summarizing, for a constant operating frequencysituation, the unloaded inverter hasa high peak thyristor anode voltageand produces higher peak currents as compared to the loadedinverter.This is obviously an undesirable situation since the currentstresses, voltage stresses, and circuit losses are highest at a'timewhen the circuit is in a standby mode.

By using the'constant duty cycle control circuit 40 shown in FIG. 3, theoperating frequency of the inverter is automatically-decreased whentheinduction heating coil is unloaded to hold the duty cycle t,/Tapproximately constant. The repetition rate of supplying gating pulsesto thyristor 33 is now variable and is in effect a function of the highfrequency oscillation period t,. The reduced induction coil current andthyristor anode voltage obtained for the unloaded inverter with constantduty cycle gating as opposed to constant frequency gating areillustrated in FIGS. 6a and 6b. Comparing the shaded portions of thethyristor anode voltage below and above the supply voltage E, in FIGS.5b and 6b, it is seen that the voltage during the high frequencyoscillation period t, is the same, but is considerably reduced in theconstant duty cycle case due to the inuch greater length of thenon-oscillatory interval T4,. Since the initial voltage V1 oncommutating capacitor 32 is considerably reduced in the constant dutycycle case, the peak amplitude of the induction coil current is alsoconsiderably less. With a constant duty cycle control circuit, the peakvoltages and peak currents for the unloaded inverter have approximatelythe same values as for the loaded inverter, and depending upon theparticular inverter and utensil can be considerably less. Optimumoperation is achieved by automatically decreasing the operatingfrequency when the induction heating coil is unloaded, which results inconsiderably lower circulating currents, thyristor voltages, and standbylosses. Conversely, the operating frequency increases automatically whenthe coil is loaded resulting in more power being coupled into theutensil. Using this technique, lower voltage and lower current ratedthyristors and other circuit components may be used for a givenapplication. For utensils made of a metal, such as stainless steel, withsuperior coupling characteristics as compared to aluminum foil, theeffect of using the constant duty cycle control circuit is even morepronounced due to the greater difference between the high frequencyoscillation period t, in the loaded and unloaded cases.

Referring to FIG. 3, constant duty cycle control circuit 40 is connectedbetween d-c input terminals 30 and 31, has an output connection to thegate of thyristor 33, and as a novel feature has a sensor inputconnection to an appropriate point on theinverter power circuit such aspoint A to sense a power circuit parameter indicative of the intervalsof conduction and nonconduction of the solid state power devices, orindicative of the oscillating and non-oscillating intervals of the powercircuit. The thyristor anode voltage, or more generally theanode-cathode voltage, is such a power circuit parameter. The majorcomponents of the preferred formof constant duty cycle control circuit40 to be described are illustrated in simplified block diagram form.Although not essential to the practice of the invention, it is desirablethat a substantial portion of the control circuit, if not the completecircuit, be fabricated as a monolithic or hybrid integrated circuit. Thethyristor anode voltage sensor 41 is more particularly an on-off sensorthat operates continuously during each complete cycle of the inverterpower circuit. The control circuit is energized for signal leveloperation by a low voltage d-c power supply 42 which nonnally derivesits power by connection between terminals 30 and 31. The remainder ofthe control circuitincludes essentially an averaging filter circuit 43and a voltage con trolled oscillator and pulse generator circuit 44.Preferably, thyristor anode voltage sensor 41 is provided by a simplevoltage divider connected between point A on the power circuit and thenegative d-c terminal that controls a switching device such as atransistor. Accordingly, the voltage divider is provided with anenergizing voltage when thyristor 33 and diode 34 are in thenonconducting state, so that the thyristor is turned on, and has noenergizing voltagewhen either power device is in the conducting state,so that the transistor is turned off. The output of thecontinuously'operating on-off sensor 41 in the form of a train ofvariable width voltage pulses 45 is fed to the averaging filter circuit43. The output of averaging filter circuit 43 is a variable directvoltage signal indicative of the conducting and nonconducting intervalsof thyristor 33 and diode 34, or of the oscillating and non-oscillatingcondition of the power circuit. This d-c leveleffectively controls therepetition rate of the gating signals generated by voltage controlledoscillator and pulse generator 44. These gating signals are also knownas tum-on signals or firing signals. A closed feedback loop control isestablished such that the repetition rate of the gating signals, andtherefore the operating frequency of the inverter, increases when theconducting intervals of the solid state power devicesdecrease, or theoscillating intervals of the power circuit decrease, and vice versa.This operation achieves an approximately constant duty cycle control.

A minimum frequency control to prevent operation of the inverter in theaudible range below about 18 kHz and also to aid the circuit to startproperly is provided in schematic form by diodes 46 and 47 and a V,,,,-,reference voltage source. The variable d-c output of averaging filtercircuit 43 is normally conducted through diode 46 to the input ofvoltage controlled oscillator 44. Diode 47 connected between thereference voltage source and the input of voltage controlled oscillator44 is ordinarily reverse biased. Diode 47 conducts, however, to provide'a predetermined minimum voltage input to voltage controlled oscillator44 whenever the d-c output of averaging filter circuit 43 falls belowthe predetermined level, at which time diode 46 becomes reverse biased.This minimum frequency control is operative during the circuit start-upand to ensure noiseless operation at all times. Although not hereillustrated, a maximum frequency control to prevent the operatingfrequency from rising above a selected upper limit of kHz or less isalso desirable. Another feature of control circuit 40 normally requiredis provision for controlled circuit shut-down.

FIG. 7 shows the detailed circuit diagram of the preferred embodiment ofconstant duty cycle control circuit 40. Low voltage d-c power supply 42is connected between the power circuit d-c input terminals 30 and 31 andincludes one or more voltage dropping resistors 50 connected in serieswith the parallel combination of a first filter capacitor 51, a secondfilter capacitor 52, and a Zener diode 53. The power circuit inputvoltage E is typically 150 volts, while the Zener regulated voltagesupplied between low voltage input terminals 54 and 31 is typically12-15 volts. Filter capacitor 51 is an energy storage electrolyticcapacitor while filter capacitor 52 is a high frequency capacitor toprovide a low source of impedance so that there is sufficient energystorage to generate a gating pulse of the required magnitude and risetime. The basic functional components of voltage controlled oscillatorand pulse generator 44 are a complementary unijunctiontransistorrelaxation oscillator for generating a train of variable repetition rategating pulses that are amplified by a pulse amplifier. The timingcircuit connected between low voltage terminals 54 and 31 includes atiming capacitor 55 connected in series with the collector-emitter pathof an n-p-n control transistor 56, an emitter resistor 57, and apotentiometer 58. The base terminals of complementary unijunctiontransistor (CUJT) 59 are respectively connected in series with baseresistors 60 and 61 between the low voltage terminals, and the emitterof CUJT 59 is connected directly to the negative terminal of timingcapacitor 55. The complementary unjunction transistor is similar to theordinary unijunction transistor butoperates in the third quadrant ratherthan in the first quadrant, and has excellent frequency stability in anoscillator. Control transistor 56 functions essentially as-a currentsource in the charging circuit for timing capacitor 55 by virtue of thefact that the collector current is a function of the base voltage. Thed-c voltage level on a control capacitor 62 connected between the baseof transistor 56 and terminal 31 thus determines the rate of charging oftiming capacitor 55 and hence the pulse generator repetition rate.Potentiometer 58 varies the resistance in the charging circuit and canbe used either as a factory adjustment or can be connected lid foradjustment by control knob 21 (FIG. 2) to provide a user control for theoutput power over a limited range. In operation, timing capacitor 55repetitively charges negatively through transistor 56, resistor 57, andpotentiometer 58, and CUJT 59 breaks over and conduts in each chargingcycle when the emitter peak point voltage is reached. Capacitor 55 thendischarges through base resistor 60 and the emitter-base junction ofp-n-p transistor 63 and generates a gating pulse. The repetition rate ofpulse generation, assuming that potentiometer 58 is set to a fixedposition, depends on the voltage level on control capacitor 62, which isa relatively large capacitor (for example, 1 microfarad) so that therepetition rate ramps from one value to another as the utensil is placedon or removed from the energized coil.

The gating pulse amplifier comprises a transistor amplifier 63 connectedin series with voltage divider resistors 641 and between terminals 54and 31, with the base of transistor 63 being coupled directly to thebase 1 of CUJT 59. The junction of resistors 64 and 65 is connected to atransistor amplifier in the Darlington configuration provided bytransistors 66 and 67. The collectors of both of these transistors arecoupled directly to positive low voltage terminal 54, while the emitterof transistor 66 is connected through an emitter resistor 68 to negativeterminal 31 and the emitter of transistor 67 is coupled to the gate ofpower thyristor 33. The gating pulse generated by CUJT 59 is invertedand amplified by transistor 63, and is further amplified by Darlingtontransistors 66 and 67. Emitter resistor 68 assures a rapid turn-off ofthe Darlington configuration comprising transistors 66 and 67 when thepulse is completed. The pulse amplifier circuit must apply asufficiently large gating pulse to properly turn on the power thyristor,and in the case of a GE C139N silicon controlled rectifier, the gatingpulse is 1 ampere with a rise time of 0.1 microsecond or faster whichlasts for at least 2 microseconds to ensure uniform turn-on of thedevice. The use of high frequency filter capacitor 52, it will berecalled, provides a low source impedance to supply the required gatingpulse.

As was mentioned, thyristor anode voltage sensor 41 is connecteddirectly between point A on the inverter power circuit, or some otherappropriate point, and negative d-c terminal 31. Although other suitabletypes of voltage responsive on-off sensors can be utilized, thepreferred sensor for this application is comprised by a resistivevoltage divider including series connected resistors 7t and '71 and anassociated signal level transistor 72. Resistor 70 is a relatively highresistance resistor, while resistor 7 ll ordinarily has a resistanceseveral magnitudes smaller. The base of transistor 72 is connected tothe junction of voltage divider resistors 70 and 71 and its emitter isconnected to negative terminal 31. Transistor 72 operates in theswitching mode, and is conductive when the thyristor anode voltage atpoint A is high with respect to terminal 31 and is nonconductive whenthyristor 33 or diode 34 are conducting so that the potentials at pointA and at negative d-c terminal 31 are approximately the same. Thus, thecollector voltage of transistor 72 is low during the nonconductingintervals of power devices 33 and 34, and high during the conductingintervals. To invert the polarity of this train of variable width squarewave voltage pulses, the collector of transistor 72 is connected to thejunction of a pair of series resistors 73 and 7 1 connected between lowvoltage terminals 54 and 31, this junction further being connected tothe base of a second transistor 75. It is seen that transistor 75conducts when transistor 72 is nonconducting, and vice versa.Accordingly, the collector voltage of transistor 75 is low during theconducting intervals of power devices 33, 34 and high during thenonconducting intervals of power devices 33, 34. r

The train of square wave voltage pulses, each with a width dependingupon the loading of the inverter power circuit, is converted to anaverage d-c voltage level by the averaging filter circuit 43. Thisfunction is provided by a resistor 76 connected between the collector oftransistor 75 and the positive plate of control capacitor 62, and by theRC charging circuit connected between terminals 54 and 31 comprised by aresistor 77, a diode 78, and control capacitor 62. The voltage level oncontrol capacitor 62, it is repeated, determines the repetition rate ofthe gating pulses. When transistor 75 is in the off state,control'capacitor 62 charges through resistor 77 and diode 78 to avoltage approaching that of positive low voltage terminal 54. Theperiodic turn-onof transistor-75, however, tends to discharge capacitor62 throughresistor 76. The value of the components is such that the rateof discharge is close to the charging rate. Furthermore, the collectorof transistor 75 is connected to'the junction between charging resistor77 and diode 78, and shunts the charging current. Accordingly, thevoltage on control capacitor 62 goes lower with increasing intervals ofconduction of transistor 75 and goes higher as the conducting intervalsbecome shorter.

To provide a maximum frequency control, a Zener diode 79 is connecteddirectly in parallel with control capacitor 62. As the voltage oncontrol capacitor 62 increases, thereby increasing the gating pulserepetition rate, a voltage is reached at which Zener diode 79 begins toconduct so that the voltage on control capacitor 62 is effectivelyclamped. The minimum frequency control includes a resistor 80 and a pairof diodes 81 and 82 connected between low voltage terminals 54 and 31,and a third diode 83 connected between the positive plate of controlcapacitor 62 and the junction of resistor 80 and the first diode 81.Diodes 81 and 82 always conduct, thus establishing the potential at theanode of diode 83 at two diode drops or approximately 1.5 volts. Diode83 is therefore forward biased whenever the voltage on control capacitor62 is below this value, such as during start-up of .the circuit, andprevents the voltage on control capacitor 62 from going below thisminimum level. The minimum gating pulse repetition rate, as previouslymentioned, is about 18 kHz for noiseless operation.

Additional circuitry controls the cooking appliance during start-up andshut-down as the user actuates the on-off control knob 21 (FIG. 2). Uponapplying power to the unit, the generation of gating pulses is inhibitedfor a short starting interval to allow the d-c input voltage E,,, tobuild up to its full value and to permit commutating capacitor 32 tocharge so as to have sufficient energy to effect proper commutation. Thestart-up and shut-down circuit includes a voltage dropping resistor 85connected in series with a capacitor 96 and a diode 87 between the powercircuit d-c input terminals 30 and 31. The base-emitter of a transistor88 shunts the blocking diode 87 to thereby permit current flow in bothdirections, and the collector of transistor 88 is connected to thejunction bwtween the voltage divider resistors and 71 in the thyristoranode voltage sensor. Further, a clamping transistor 89 is connecteddirectly across the terminals of timing capacitor 55 in voltagecontrolled oscillator and pulse generator 44, the base of clampingtransistor 89 being connected through a resistor 90 to the positiveterminal of capacitor 86. To provide a rapid discharge path forcapacitor 86 during circuit shut-down, a diode 91 connected to itspositive terminal and to low voltage supply terminal 54 acts inconnection with the previously mentioned blocking diode 87. Theoperation of the start-up and shut-down circuit will be explained inconjunction with a brief review of the operation of the entire constantduty cycle control circuit 40.

It is assumed in the following description that the power level controlknob 21' adjuting potentiometer 58 has been set to the desired positionand that the TV dinner 17 has not yet been placed in cooking position onthe warming appliance when the unit is turned on. Upon applying power,capacitor 86 begins to charge through resistor 85 and the base-emitterjunction of transistor 88. Since transistor 88 is conducting, the baseof transistor72 in thyristor anode voltage sensor 41 is clamped tonegative terminal 31 to thereby inhibit operation of the sensor duringthe starting interval. Clamping transistor 89 also conducts and inhibitsthe complementary unijunction transistor oscillator since timingcapacitor 55 cannot charge. When capacitor 86 becomes nearly fullycharged, the potential at its positive terminal biases clampingtransistor 89 to the nonconducting state, and transistor 88 turns off,thereby releasing voltage controlled oscillator and pulse generator 44and thyristor anode sensor 41 for operation. In the meantime, controlcapacitor 62 has charge rapidly through resistor 80 and diode 83 in theminimum frequency control circuit to reach the minimum frequency controlvoltage. constant duty cycle control circuit 40 now begins to generategating pulses at a repetition rate at approximately the minimumfrequency of 18 kHz. In the voltage controlled oscillator, CUJT 59breaks over and conducts repetitively as the voltage on timing capacitor55 reaches the emitter peak point voltage. The gating pulse is amplifierby transistor 63, and also by the Darlington transistor amplifier 66, 67before being applied to the gate electrode of power thyristor 33. Withthe inverter in the unloaded condition, the losses in the high frequencyoscillatory circuit are low, with the result that the period t, isrelatively long as shown in FIG. 6a. Assuming steady state operation,thyristor anode voltage sensor 41 operates to produce at the collectorof transistor 72 a train of variable width square wave voltage pulseswhose polarity is inverted by transistor 75. Whereas transistor 72 isturned on during the nonconducting intervals of power thyristor 33 andpower diode 34, transistor is turned on during their conductingintervals. Averaging filter circuit 43 converts the square orquasi-square wave output voltage of thyristor anode voltage sensor 41 toa d-c voltage level indicative of the intervals of conduction and nonconduction of the solid state power devices in the inverter powercircuit. This corresponds to the periods of oscillation andnon-oscillation of the power circuit. The circuit components areselected such that with the coil unloaded, control capacitor 62 has theproper voltage so that the gating frequency of the inverter is of thecorrect value to produce the desired duty cycle. A duty cycle near 50percent is a practical value.

Utilizing a closed feedback loop control technique, control circuit 40maintains an approximately constant duty cycle t,/T when the TV dinner17' is placed in cooking position on the warming appliance. In theloaded condition of the inverter, as opposed to the unloaded condition,the losses in the high frequency oscillation circuit are higher and theperiod t, is shorter. Thyristor anode voltage sensor 41 senses theshorter intervals of conductor and nonconduction of power thyristor 33and power diode 34, with the effective result that transistor 75conducts periodically for shorter intervals so that control capacitor 62charges to a higher voltage. Consequently, the repetition rate of thegating pulses generated by voltage controlled oscillator and pulsegenerator 44 is higher. Since the operating frequency of the inverterpower circuit is increased, and the period T is now shorter, the dutycycle t,/T is approximately the same as in the unloaded case. Since theloop gain of the control loop is relatively high, the loaded andunloaded duty cycles need only be different by a small amount to affordthe necessary change in voltage on control capacitor 62 which isnecessary to achieve the desired frequency variations. That is, it takesonly a small change in voltage on control capacitor 62 to vary thegating frequency over a wide range. Therefore, for all practicalpurposes the duty cycle remains the same in the loaded and unloadedcase. The act of placing the utensil on the inductor warming applianceis relatively long compared to the periods of the ultrasonic operatingfrequencies, and hence the operating frequency ramps from one value tothe other. Upon turning off the unit, as'was previously explained,capacitor 86 discharges rapidly through diodes 91 and 87. Diode 91provides for a fast discharge path hrough resistor 50 and the collapsingd-c power supply. Thus, capacitor 86 is charged slowly at turnon throughresistor 85and'is rapidly discharged at turn-off through resistor 50which is a much smaller resistance than resistor 85. In this manner thecircuit can be restarted again without having to wait for a considerabledischarge time.

An advantage of the inverter control circuit constructed according tothe teaching of the invention is that the detection of the presence andabsence of a utensil load by sensing a power circuit voltage or otherappropriate power circuit parameter is in keeping with the principle ofa smooth-top appliance. The placing of holes-in utensil support 16 orthe use of a sensor that projects'above the utensil support is notrequired. The on-off sensor, furthermore, does not conduct powercurrents. Although this may increase the complexity of the controlcircuit, the trade off is made of keeping the power circuit as simple aspossible to help achieve the low costs often needed in consumer-orientedappliances. As compared to a threshold type sensor, the onoff sensorallows lower peak voltage and current stresses.

In summary, a constant duty cycle control circuit as herein describedautomatically adjusts the operating frequency of the inverter powercircuit according to the load conditions. To compensate for theincreased period of oscillation when the inverter changes from theloaded to the unloaded condition, the operating frequency is decreasedto achieve more optimum performance. In the case of induction cookingappliances,

lower current rated and voltage rated power semiconductors and othercircuit components can be used since the decreased operating frequencywith the utensil removed from the coil reduces the electrical stressesand losses. Conversely, the operating frequency increases when loaded tosupply more power to the load. The novel on-off sensor is operativecontinuously to supply data as to the load condition of the inverterpower circuit. The inverter with constant duty cycle control can be usedfor other induction heating applications, and for appropriateapplications not related to induction heating.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A solid state cooling appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken utensil support and producing an alternatingmagnetic field,

a static power conversion circuit including a variable operatingfrequency power circuit controlled by solid state power device means fordriving said induction heating coil, and

a constant duty cycle control circuit operating at signal level andconnected to selected points on said power circuit, said constant dutycycle control circuit supplying turn-on signals to said solid statepower device means to continuously vary said operating frequency as afunction of the load condition of said induction heating coil, in whichsaid constant duty cycle control circuit is controlled by a sensorconnected between a pair of said points on said power circuit'to sense apower circuit parameter indicative of the intervals of conduction andnonconduction of said solid state power device means.

2. A cooking appliance according to claim 1 in which said sensor is anon-off sensor for sensing the voltage at one terminal of said solidstate power device means.

3. A cooking appliance according to claim 2 in which said on-off sensorcomprises a voltage divider circuit and a solid state switch coupled toa junction of said voltage divider circuit for generating a train ofvariable width voltage pulses.

4. A solid state cooking appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken utensil support and producing an alternatingmagnetic field,

a static power conversion circuit including a variable operatingfrequency power circuit controlled by solid state power device means fordriving said induction heating coil, and

a constant duty cycle control circuit operating at signal level andconnecting to selected points on said power circuit, said constant dutycycle control circuit supplying tum-on signals to said'solid state powerdevice means to continuously vary said operating frequency as a functionof the load condition of said induction heating coil, in which saidconstant duty cycle control circuit comprises an on-off sensor connectedbetween a pair of said points on said power circuitfor generating atrain of variable width pulses indicative of the intervals of conductionand nonconduction of said solid state power device means, an averagingfilter circuit for converting saidtrain of pulses to a variable directvoltage signal, and a voltage controlled oscillator and pulse generatorfor producing said turnonsignals at. a variable repetition ratedependent .on the magnitude of said direct voltage signal. 5. A cookingappliance according to claim 4 in which said constant duty cycle controlcircuit further includes a minimum and maximum frequency control circuitfor limiting the magnitude of said direct voltage signal to apredetermined range.

A 6. A solid st ate cooking appliance for inductively heating a utensilload comprising w an induction heating coil mounted adjacent asubstantially unbroken utensil support and producing an alternatingmagnetic field,

a static power conversion circuit including a variable operatingfrequency power circuit controlled by solid state power device means fordriving said induction heating coil, and

a constant duty cycle control circuit operating at signal level andconnected to selected points on said power circuit, said constant dutycycle control circuit supplying turn-on signals to said solid statepower device means to continuously vary said operatingfrequency as afunction of the load condition of said induction heating coil, in whichsaid variable operating frequency power circuit is an oscillatory powercircuit, and

said constant duty cycle control circuit comprises an on'off sensorconnected between a pair of said points on said power circuit forgenerating a train of variable width pulses indicative of the intervalsof oscillation and n'onoscillation of said power circuit, an averagingfilter circuit for converting said train of pulses to a variable directvoltage signal, and a voltage controlled oscillator and pulse generatorfor producing said turn-on signals at a variable repetition ratedependent on the magnitude of said direct voltage signal.

7. A cooking appliance according to claim 6 in which said constant dutycycle control circuit further includes a minimum and maximum frequencycontrol circuit for limiting the magnitude of said direct voltage signalto a predetermined range.

8. A cooking appliance according to claim 7 in which said constant dutycycle control circuit further includes a start-up circuit operativelycoupled to inhibit said onoff sensor and voltage control oscillatorduring a starting interval.

9. A solid state cooking appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken non-metallic utensil support for producing analternating magnetic field,

a variable operating frequency inverter power circuit controlled bysolid state power device means for converting an input unidirectionalvoltage to an ultrasonic frequency wave that drives said inductionheating coil, and

a signal level constant duty cycle control circuit controlled by anon-off sensor connected between selected points on said inverter powercircuit to sense a predetermined power circuit parameter,

said constant duty cycle control circuit supplying gating signals tosaidsolid state power device means to continuously vary said operatingfrequency in accordance with the presence and absence of a utensilcoupled with said induction heating coil, in which said on-off sensor isconnected to sense the intervals of conduction and no'nconduction ofsaid solid state power device means,

said constant duty cycle control circuit operating to increase saidoperating frequency with the sensing of shorter conduction intervals dueto the coupling of power to the utensil, and to decrease said operatingfrequency with the sensing of longer conduction intervals due to theabsense of the utensil.

10. A solid state cooking appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken non-metallic utensil support for producing analternating magnetic field,

a variable operating frequency inverter power circuit controlled bysolid state power device means for converting an input unidirectionalvoltage to an 111-. trasonic frequency wave that drives said inductionheating coil, and

a signal level constant duty cycle control circuit controlled by anon-off sensor connected between selected points on said inverter powercircuit to sense a predetermined power circuit parameter,

said constant duty cycle control circuit supplying gating signals tosaidsolid state power device means to continuously vary said operatingfrequency in accordance with the presence and absence of a utensilcoupled with said induction heating coil, in which said inverter powercircuit is a series resonant circuit and said solid state power devicemeans comprises the inverse-parallel combination of a thyristor and adiode, and

said on-off sensor connected between selected points on said inverterpower circuit is a voltage responsive sensor connected to one terminalof said thyristor for sensing the intervals of conduction andnon-conduction of said thyristor-diode combination.

11. A cooking appliance according to claim 10 in which said on-offsensor is comprised by a resistance voltage divider circuit and a solidstate switch coupled to a junction of said voltage divider to produce atrain of variable width voltage pulses.

12. A cooking appliance according to claim 11 in which said constantduty cycle control circuit further includes an averaging filter circuitfor converting said train of pulses to a variable direct voltage signal,and a voltage controlled oscillator and pulse generator for producingsaid gating signals at a variable repetition rate depending on themagnitude of said direct voltage signal.

1. A solid state cooling appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken utensil support and producing an alternatingmagnetic field, a static power conversion circuit including a variableoperating frequency power circuit controlled by solid state power devicemeans for driving said induction heating coil, and a constant duty cyclecontrol circuit operating at signal level and connected to selectedpoints on said power circuit, said constant duty cycle control circuitsupplying turn-on signals to said solid state power device means tocontinuously vary said operating frequency as a function of the loadcondition of said induction heating coil, in which said constant dutycycle control circuit is controlled by a sensor connected between a pairof said points on said power circuit to sense a power circuit parameterindicative of the intervals of conduction and nonconduction of saidsolid state power device means.
 2. A cooking appliance according toclaim 1 in which said sensor is an on-off sensor for sensing the voltageat one terminal of said solid state power device means.
 3. A cookingappliance according to claim 2 in which said on-off sensor comprises avoltage divider circuit and a solid state switch coupled to a junctionof said voltage divider circuit for generating a train of variable widthvoltage pulses.
 4. A solid state cooking appliance for inductivelyheating a utensil load comprising an induction heating coil mountedadjacent a substantially unbroken utensil support and producing analternating magnetic field, a static power conversion circuit includinga variable operating frequency power circuit controlled by solid statepower device means for driving said induction heating coil, and aconstant duty cycle control circuit operating at signal level andconnecting to selected points on said power circuit, said constant dutycycle control circuit supplying turn-on signals to said solid statepower device means to continuously vary said operating frequency as afunction of the load condition of said induction heating coil, in whichsaid constant duty cycle control circuit comprises an on-off sensorconnected between a pair of said points on said power circuit forgenerating a train of variable width pulses indicative of the intervalsof conduction and nonconduction of said solid state power device means,an averaging filter circuit for converting said train of pulses to avariable direct voltage signal, and a voltage controlled oscillator andpulse generator for producing said turn-on signals at a variablerepetition rate dependeNt on the magnitude of said direct voltagesignal.
 5. A cooking appliance according to claim 4 in which saidconstant duty cycle control circuit further includes a minimum andmaximum frequency control circuit for limiting the magnitude of saiddirect voltage signal to a predetermined range.
 6. A solid state cookingappliance for inductively heating a utensil load comprising an inductionheating coil mounted adjacent a substantially unbroken utensil supportand producing an alternating magnetic field, a static power conversioncircuit including a variable operating frequency power circuitcontrolled by solid state power device means for driving said inductionheating coil, and a constant duty cycle control circuit operating atsignal level and connected to selected points on said power circuit,said constant duty cycle control circuit supplying turn-on signals tosaid solid state power device means to continuously vary said operatingfrequency as a function of the load condition of said induction heatingcoil, in which said variable operating frequency power circuit is anoscillatory power circuit, and said constant duty cycle control circuitcomprises an on-off sensor connected between a pair of said points onsaid power circuit for generating a train of variable width pulsesindicative of the intervals of oscillation and nonoscillation of saidpower circuit, an averaging filter circuit for converting said train ofpulses to a variable direct voltage signal, and a voltage controlledoscillator and pulse generator for producing said turn-on signals at avariable repetition rate dependent on the magnitude of said directvoltage signal.
 7. A cooking appliance according to claim 6 in whichsaid constant duty cycle control circuit further includes a minimum andmaximum frequency control circuit for limiting the magnitude of saiddirect voltage signal to a predetermined range.
 8. A cooking applianceaccording to claim 7 in which said constant duty cycle control circuitfurther includes a start-up circuit operatively coupled to inhibit saidon-off sensor and voltage control oscillator during a starting interval.9. A solid state cooking appliance for inductively heating a utensilload comprising an induction heating coil mounted adjacent asubstantially unbroken non-metallic utensil support for producing analternating magnetic field, a variable operating frequency inverterpower circuit controlled by solid state power device means forconverting an input unidirectional voltage to an ultrasonic frequencywave that drives said induction heating coil, and a signal levelconstant duty cycle control circuit controlled by an on-off sensorconnected between selected points on said inverter power circuit tosense a predetermined power circuit parameter, said constant duty cyclecontrol circuit supplying gating signals to said solid state powerdevice means to continuously vary said operating frequency in accordancewith the presence and absence of a utensil coupled with said inductionheating coil, in which said on-off sensor is connected to sense theintervals of conduction and nonconduction of said solid state powerdevice means, said constant duty cycle control circuit operating toincrease said operating frequency with the sensing of shorter conductionintervals due to the coupling of power to the utensil, and to decreasesaid operating frequency with the sensing of longer conduction intervalsdue to the absense of the utensil.
 10. A solid state cooking appliancefor inductively heating a utensil load comprising an induction heatingcoil mounted adjacent a substantially unbroken non-metallic utensilsupport for producing an alternating magnetic field, a variableoperating frequency inverter power circuit controlled by solid statepower device means for converting an input unidirectional voltage to anultrasonic frequency wave that drives said induction heating coil, and asignal level constant dutY cycle control circuit controlled by an on-offsensor connected between selected points on said inverter power circuitto sense a predetermined power circuit parameter, said constant dutycycle control circuit supplying gating signals to said solid state powerdevice means to continuously vary said operating frequency in accordancewith the presence and absence of a utensil coupled with said inductionheating coil, in which said inverter power circuit is a series resonantcircuit and said solid state power device means comprises theinverse-parallel combination of a thyristor and a diode, and said on-offsensor connected between selected points on said inverter power circuitis a voltage responsive sensor connected to one terminal of saidthyristor for sensing the intervals of conduction and non-conduction ofsaid thyristor-diode combination.
 11. A cooking appliance according toclaim 10 in which said on-off sensor is comprised by a resistancevoltage divider circuit and a solid state switch coupled to a junctionof said voltage divider to produce a train of variable width voltagepulses.
 12. A cooking appliance according to claim 11 in which saidconstant duty cycle control circuit further includes an averaging filtercircuit for converting said train of pulses to a variable direct voltagesignal, and a voltage controlled oscillator and pulse generator forproducing said gating signals at a variable repetition rate depending onthe magnitude of said direct voltage signal.